Reducing the size of germanium islands improves their optical properties. The spatially strong localised holes in the islands are more uncertain in momentum and thereby the probability of direct optical transitions is enhanced. Carbon predepositon onto Si has been shown to reduce the size of germanium islands. We have studied such islands grown by molecular beam epitaxy. The germanium islands show an intense PL-signal, which is typically more than one order of magnitude larger in intensity than the PL from silicon-germanium quantum wells. Reducing the amount of Ge from 3.4 monolayer (ML) to 1.0 hit results in a PL blueshift from 0.81 to 1.08 eV in conjunction with a narrowing of the PL peak. The blueshift indicates a reduction in the size of the germanium islands, which was confirmed by TEM. increasing the C coverage from 0.2 to 0.3 ML leads to a red shifted FL. This is attributed to a change in the aspect ratio of the islands with the result of an increased vertical island height.